CONVERTIBLE NOTES PAYABLE - Composition of Notes (Details) - Senior Notes - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018	Sep. 10, 2018
Senior Notes Due 2025
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	2.50%		2.50%
Long-term Debt, Gross	$ 276,000	$ 276,000
Unamortized debt discount	(68,245)	(74,836)
Unamortized debt issuance costs	(5,400)	(6,073)
Long-term Debt	$ 202,355	195,091
Convertible Notes Due 2019
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	4.50%
Long-term Debt, Gross		22,590
Unamortized debt discount		(125)
Unamortized debt issuance costs		(8)
Long-term Debt		$ 22,457

Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

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Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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